Imaging lens

ABSTRACT

An imaging lens includes a first lens group and a second lens group, arranged in this order from an object side to an image plane side. The first lens group includes a first lens having positive refractive power, a second lens having positive refractive power, and a third lens. The second lens group includes a fourth lens having positive refractive power, a fifth lens, and a sixth lens having negative refractive power. The fifth lens has a specific Abbe&#39;s number.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No.14/568,173, filed on Dec. 12, 2014, allowed, which claims priority ofJapanese Patent Application No. 2014-136541, filed on Jul. 2, 2014.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.In particular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a camera to be builtin a portable device including a cellular phone and a portableinformation terminal, a digital still camera, a security camera, avehicle onboard camera, and a network camera.

In these years, in place of cellular phones that are intended mainly formaking phone calls, so-called “smartphones”, which are multi-functionalcellular phones that can be used to execute various application softwareas well as making phone calls, have been more widely used. By executingapplication software on a smartphone, the smartphone can be used, forexample, as a digital still camera, a car navigation system, or thelike. In order to execute the functions, most models of the smartphoneshave cameras, similar to the cellular phones.

Generally speaking, product groups of the smartphones are oftencategorized according to specifications thereof for beginners toadvanced users. Among them, an imaging lens to be mounted in a productdesigned for advanced users is required to have a high resolution lensconfiguration so as to be also applicable to a high pixel count imagingelement.

As a method of attaining the high-resolution imaging lens, there is amethod of increasing the number of lenses that compose the imaging lens.However, the increase of the number of lenses easily causes an increasein the size of the imaging lens. Therefore, the lens configurationhaving a large number of lenses is disadvantageous for mounting in asmall-sized camera such as the above-described smartphones. For thisreason, an imaging lens has been developed so as to restrain the numberof lenses as small as possible. However, with rapid advancement inachieving higher pixel count of an imaging element in these days, animaging lens has been developed so as to attain higher resolution ratherthan attaining a shorter total track length thereof. For example,although it has been common to mount a conventional camera unitcontaining an imaging lens and an imaging element inside of asmartphone, nowadays, there is an attempt to attach a camera unit, whichis a separate unit from the smartphone, to a smartphone, so that it ispossible to obtain an image that is equivalent to that obtained by adigital still camera.

In case of a lens configuration composed of six lenses, since the numberof lenses that compose an imaging lens is large, it is somewhatdisadvantageous for downsizing the imaging lens. However, since there ishigh flexibility in designing, it has potential of attainingsatisfactory correction of aberrations and downsizing in a balancedmanner. For example, as an imaging lens having a six-lens configurationas described above, an imaging lens described in Patent Reference hasbeen known.

Patent Reference: Japanese Patent Application Publication No.2013-195587

The imaging lens described in Patent Reference includes a first lensthat is positive and has a convex surface directing to an object side, asecond lens that is negative and has a concave surface directing to animage plane side, a third lens that is negative and has a concavesurface directing to the object side, a fourth lens and a fifth lensthat are positive and have convex surfaces to the image plane side, anda sixth lens that is negative and has a concave surface directing to theobject side. According to the imaging lens of Patent Reference, bysatisfying a conditional relation concerning a ratio between a focallength of the first lens and a focal length of the third lens and aratio between a focal length of the second lens and a focal length ofthe whole lens system, it is possible to satisfactorily correct adistortion and a chromatic aberration.

The cellular phones and smartphones have higher functions and smallersizes each year and imaging lenses are required to have even smallersizes than before. In case of the imaging lens of Patent Reference, thedistance from the object-side surface of the first lens to the imageplane of an imaging element is long. Therefore, to satisfy theabove-described demands, there is a limit by itself in satisfactorilycorrecting aberrations while downsizing the imaging lens.

Here, such a problem is not specific to the imaging lens to be mountedin the cellular phones and smartphones. Rather, it is a common problemeven for an imaging lens to be mounted in a relatively small camera suchas digital still cameras, portable information terminals, securitycameras, vehicle onboard cameras, and network cameras.

In view of the above-described problems in conventional techniques, anobject of the present invention is to provide an imaging lens that canattain downsizing thereof and satisfactory aberration correction.

Further objects and advantages of the present invention will be apparentfrom the following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a firstaspect of the present invention, an imaging lens includes a first lensgroup having positive refractive power; and a second lens group havingnegative refractive power, arranged in the order from an object side toan image plane side. The first lens group includes a first lens havingpositive refractive power, and a second lens having positive refractivepower and a third lens having negative refractive power. The second lensgroup includes a fourth lens having positive refractive power, a fifthlens, and a sixth lens having negative refractive power.

According to the first aspect of the present invention, when the wholelens system has a focal length f, the first lens has a focal length f1,the first lens has an Abbe's number νd1, the second lens has an Abbe'snumber νd2, and the third lens has an Abbe's number νd3, the imaginglens of the present invention satisfies the following conditionalexpressions (1) to (4):10<f1/f<40  (1)40<νd1<75  (2)40<νd2<75  (3)15<νd3<35  (4)

According to the first aspect of the present invention, the first lensgroup is composed of three lenses, refractive powers of which arearranged in the order of positive-positive-negative. Those three lensesare made of lens materials that satisfy the conditional expressions (2)to (4). As a result, those three lenses are composed of a combination oflenses made of low-dispersion materials and a lens made of ahigh-dispersion material. With the arrangement of the refractive powersand the order of the Abbe's numbers of the respective lenses, it ispossible to suitably restrain generation of a chromatic aberration inthe first lens group and to satisfactorily correct the chromaticaberration if generated.

According to the first aspect of the invention, in the imaging lens, thepositive refractive power is shared between the two lenses, the firstlens and the second lens. Therefore, it is achievable to restrain therespective refractive powers of the first lens and the second lens torelatively small values, and it is achievable to suitably downsize theimaging lens while satisfactorily correcting aberrations.

When the imaging lens satisfies the conditional expression (1), it isachievable to satisfactorily correct an astigmatism and a fieldcurvature, while downsizing the imaging lens. When the value exceeds theupper limit of “40”, the first lens has relatively weak positiverefractive power relative to the refractive power of the whole lenssystem. As a result, the second lens has relatively strong positiverefractive power in the first lens group. Therefore, an image-formingsurface curves to the object side, i.e., the field curvature isinsufficiently corrected. In addition, an astigmatic differenceincreases, so that it is difficult to obtain satisfactory image-formingperformance.

On the other hand, when the value is below the lower limit of “10”, thefirst lens has relatively strong positive refractive power relative tothe refractive power of the whole lens system. Therefore, it isadvantageous for downsizing of the imaging lens. However, a back focallength is short, so that it is difficult to secure space for disposingan insert such as an infrared cutoff filter. Moreover, the astigmatismincreases, and a coma aberration for off-axis light flux increases, sothat it is difficult to obtain satisfactory image-forming performance.

According to a second aspect of the invention, when the first lens grouphas a focal length F1, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(5):0.7<F1/f<1.2  (5)

When the imaging lens satisfies the conditional expression (5), it isachievable to restrain the chromatic aberration and the astigmatismwithin satisfactory ranges, while downsizing the imaging lens. Inaddition, when the imaging lens satisfies the conditional expression(5), it is also achievable to restrain the incident angles of lightbeams emitted from the imaging lens to the image plane within the rangeof a chief ray angle (CRA). As is well known, an imaging element such asa CCD sensor or a CMOS sensor has a so-called chief ray angle (CRA) setin advance, i.e. a range of an incident angle of a light beam that canbe taken in the sensor. By restraining the incident angle of a lightbeam emitted from the imaging lens to the image plane within the rangeof CRA, it is possible to suitably restrain generation of so-called“shading”, which is a phenomenon that the image periphery becomes dark.

When the value exceeds the upper limit of “1.2” in the conditionalexpression (5), the first lens group has weak refractive power relativeto that of the whole lens system. Therefore, although it is advantageousfor correction of an axial chromatic aberration, it is difficult todownsize the imaging lens. In addition, since the astigmatic differenceincreases at the image periphery, it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “0.7”, thefirst lens group has strong refractive power relative to that of thewhole lens system, so that it is advantageous for downsizing of theimaging lens. However, the axial chromatic aberration is insufficientlycorrected (a focal position at a short wavelength moves to the objectside relative to a focal position at a reference wavelength). At thesame time, a chromatic aberration of magnification is excessivelycorrected (an image-forming point at a short wavelength moves in adirection to be away from an optical axis relative to an image-formingpoint at a reference wavelength). Moreover, since the field curvature isinsufficiently corrected, so that it is difficult to obtain satisfactoryimage-forming performance. Furthermore, it is also difficult to restrainan incident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA.

According to a third aspect of the invention, when the second lens has afocal length f2, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(6):0.3<f2/f<0.9  (6)

When the imaging lens satisfies the conditional expression (6), it isachievable to restrain the astigmatism and the chromatic aberration ofmagnification within satisfactory ranges, while downsizing the imaginglens. When the value exceeds the upper limit of “0.9”, the second lenshas weak refractive power relative to that of the whole lens system, andthe first lens group has relatively weak positive refractive power. Forthis reason, although it is advantageous for securing the back focallength, it is difficult to downsize the imaging lens.

As described above, the first lens group includes the first and thesecond lenses, which have positive refractive powers, and the third lenshaving negative refractive power. When the value exceeds the upper limitin the conditional expression (6), while the first lens group hasrelatively weak positive refractive power, the third lens has relativelystrong negative refractive power. In order to satisfactorily correctaberrations in the first lens group, it is necessary to weaken therefractive power of the third lens. However, when the third lens hasweak refractive power, the axial chromatic aberration is insufficientlycorrected, and the astigmatic difference increases. Therefore, when thevalue exceeds the upper limit, it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “0.3” inthe conditional expression (6), the second lens has strong refractivepower relative to the whole lens system, and the first lens group hasrelatively strong positive refractive power. Therefore, although it isadvantageous for downsizing of the imaging lens, it is difficult tosecure the back focal length. Here, when the value is below the lowerlimit, it is necessary to increase the refractive power of the thirdlens in order to satisfactorily correct the aberrations in the firstlens group. In such a case, although it is advantageous for correctingthe axial chromatic aberration, the chromatic aberration ofmagnification for an off-axis light flux is excessively corrected.Therefore, also when the value is below the lower limit, it is difficultto obtain satisfactory image-forming performance.

According to a fourth aspect of the invention, when the second lens hasa focal length f2 and the third lens has a focal length f3, the imaginglens having the above-described configuration preferably satisfies thefollowing conditional expression (7):−5<f3/f2<−1  (7)

When the imaging lens satisfies the conditional expression (7), it isachievable to satisfactorily correct the chromatic aberration and aspherical aberration, while downsizing the imaging lens. When the valueexceeds the upper limit of “−1”, the third lens has strong negativerefractive power relative to the positive refractive power of the secondlens. Therefore, although it is advantageous for securing the back focallength, it is difficult to downsize the imaging lens. In addition, sincea spherical aberration increases, it is difficult to obtain satisfactoryimage-forming performance. On the other hand, when the value is belowthe lower limit of “−5”, although it is advantageous for downsizing ofthe imaging lens, the axial chromatic aberration is insufficientlycorrected. Therefore, also in this case, it is difficult to obtainsatisfactory image-forming performance.

According to a fifth aspect of the invention, when the first lens grouphas a focal length F1 and the second lens group has a focal length F2,the imaging lens having the above-described configuration preferablysatisfies the following conditional expression (8):−12<F2/F1<−1.5  (8)

When the imaging lens satisfies the conditional expression (8), it isachievable to restrain the astigmatism, the field curvature, and thechromatic aberration within preferable ranges in a balanced manner,while downsizing the imaging lens. When the value exceeds the upperlimit of “−1.5”, it is advantageous for downsizing of the imaging lens.However, the axial chromatic aberration is insufficiently corrected andthe chromatic aberration of magnification for an off-axis light flux atthe image periphery is excessively corrected. Moreover, the astigmaticdifference increases and the field curvature is insufficientlycorrected, so that it is difficult to obtain satisfactory image-formingperformance.

On the other hand, when the value is below the lower limit of “−12”,although it is easy to secure the back focal length, it is difficult todownsize the imaging lens. Moreover, the astigmatic differenceincreases, and the image-forming surface curves to the image plane side,i.e., the field curvature is excessively corrected. As a result, it isdifficult to obtain satisfactory image-forming performance.

According to a sixth aspect of the invention, when a distance on anoptical axis between the third lens and the fourth lens is D34, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (9):0.1<D34/f<0.4  (9)

When the imaging lens satisfies the conditional expression (9), it isachievable to restrain the distortion, the astigmatism, and the fieldcurvature within preferred ranges in a balanced manner, whilerestraining an incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “0.4”, although it is easy to restrain the incidentangle within the range of CRA, it is difficult to secure the back focallength. In addition, the astigmatism increases, and the field curvatureis excessively corrected, so that it is difficult to obtain satisfactoryimage-forming performance.

On the other hand, when the value is below the lower limit of “0.1”, aminus distortion increases and the field curvature is insufficientlycorrected. Furthermore, since the astigmatism also increases, it isdifficult to obtain satisfactory image-forming performance. Moreover, itis difficult to restrain an incident angle of a light beam emitted fromthe imaging lens to the image plane within the range of CRA.

According to a seventh aspect of the invention, when the fourth lens hasan Abbe's number νd4, the fifth lens has an Abbe's number νd5, and thesixth lens has an Abbe's number νd6, in order to more satisfactorilycorrect the chromatic aberration, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expressions (10) to (12):40<νd4<75  (10)15<νd5<35  (11)40<νd6<75  (12)

According to an eighth aspect of the invention, when the fifth lens hasnegative refractive power, the fourth lens has a focal length f4, andthe fifth lens has a focal length f5, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (13):−15<f5/f4<−5  (13)

When the imaging lens satisfies the conditional expression (13), it isachievable to satisfactorily correct the chromatic aberration ofmagnification and the field curvature. When the value exceeds the upperlimit of “−5”, the fifth lens has strong negative refractive powerrelative to the positive refractive power of the fourth lens. Therefore,the chromatic aberration of magnification for an off-axis light flux atan image periphery is excessively corrected, and the field curvature isinsufficiently corrected. As a result, it is difficult to obtainsatisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−15”, thefifth lens has weak negative refractive power relative to the positiverefractive power of the fourth lens. In order to satisfactorily correctthe aberrations, it is necessary to increase the refractive power of thesixth lens, which has also negative refractive power in the second lensgroup. However, in this case, although it is advantageous for correctingthe field curvature, the chromatic aberration of magnification at theimage periphery is excessively corrected. As a result, it is difficultto obtain satisfactory image-forming performance.

According to a ninth aspect of the invention, when a composite focallength of the fifth lens and the sixth lens is f56, the imaging lenshaving the above-described configuration preferably satisfies thefollowing conditional expression (14):−3<f56/f<−0.8  (14)

When the imaging lens satisfies the conditional expression (14), it isachievable to restrain the chromatic aberration, the distortion, and theastigmatism within preferred ranges in a balanced manner, whiledownsizing the imaging lens. When the value exceeds the upper limit of“−0.8”, the second lens group has relatively strong negative refractivepower. Although it is advantageous for downsizing of the imaging lens, aplus distortion increases and the chromatic aberration of magnificationis excessively corrected at the image periphery. As a result, it isdifficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “−3”,although it is easy to secure the back focal length, it is difficult todownsize the imaging lens. Moreover, the minus distortion increases andthe astigmatic difference increases, so that it is difficult to obtainsatisfactory image-forming performance.

According to a tenth aspect of the invention, when the sixth lens has afocal length f6 and a composite focal length of the fifth lens and thesixth lens is f56, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(15):0.7<f6/f56<1.2  (15)

When the imaging lens satisfies the conditional expression (15), it isachievable to restrain the chromatic aberration, the field curvature,and the distortion within preferred ranges in a balanced manner. As isshown in the conditional expression (15), according to the tenth aspectof the invention, the sixth lens accounts for the most part of thenegative refractive power in the second lens group. The fifth lens hasvery weak refractive power relative to that of the sixth lens. With thisconfiguration, it is achievable to perform fine correction ofaberrations in the fifth lens, and it is achievable to suitably restrainthe incident angle to the image plane within the range of CRA, inaddition to correction of the aberrations, in the sixth lens.

When the value exceeds the upper limit of “1.2” in the conditionalexpression (15), although it is advantageous for correcting the axialchromatic aberration, the minus distortion increases. Moreover, thefield curvature is insufficiently corrected, and the chromaticaberration of magnification is excessively corrected. As a result, it isdifficult to obtain satisfactory image-forming performance.

On the other hand, when the value is below the lower limit of “0.7”,although it is easy to correct the distortion, the axial chromaticaberration is insufficiently corrected, and it is difficult to obtainsatisfactory image-forming performance.

According to an eleventh aspect of the invention, when the sixth lenshas a focal length f6, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(16):−3.5<f6/f<−0.5  (16)

When the imaging lens satisfies the conditional expression (16), it isachievable to restrain the chromatic aberration, the distortion, and theastigmatism within preferred ranges in a balanced manner, whiledownsizing the imaging lens. Moreover, when the imaging lens satisfiesthe conditional expression (16), it is achievable to restrain anincident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA. When the value exceeds the upperlimit of “−0.5”, it is advantageous for correcting the axial chromaticaberration. However, the plus distortion increases, and the chromaticaberration of magnification for the off-axis light flux at the imageperiphery is excessively corrected, so that it is difficult to obtainsatisfactory image-forming performance. In addition, it is difficult torestrain an incident angle of a light beam emitted from the imaging lensto the image plane within the range of CRA.

On the other hand, when the value is below the lower limit of “−3.5”,although it is easy to restrain the incident angle to the image planewithin the range of CRA, the minus distortion increases and thechromatic aberration of magnification for the off-axis light flux at theimage periphery is insufficiently corrected. As a result, it is alsodifficult to obtain satisfactory image-forming performance.

According to the imaging lens of the present invention, it is possibleto provide a small-sized imaging lens that is especially suitable formounting in a small-sized camera, while having high resolution withsatisfactory correction of aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 1 according to an embodiment of thepresent invention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 2 according to the embodiment of thepresent invention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 shows a sectional view of a schematic configuration of an imaginglens in Numerical Data Example 3 according to the embodiment of thepresent invention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 4 according to the embodiment ofthe present invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 5 according to the embodiment ofthe present invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13;

FIG. 15 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 6 according to the embodiment ofthe present invention;

FIG. 17 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 16;

FIG. 18 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 16;

FIG. 19 shows a sectional view of a schematic configuration of animaging lens in Numerical Data Example 7 according to the embodiment ofthe present invention;

FIG. 20 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 19; and

FIG. 21 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, 13, 16, and 19 are schematic sectional views of theimaging lenses in Numerical Data Examples 1 to 7 according to theembodiment, respectively. Since the imaging lenses in those NumericalData Examples have the same basic configuration, the lens configurationof the embodiment will be described with reference to the illustrativesectional view of Numerical Data Example 1.

As shown in FIG. 1, according to the embodiment, the imaging lensincludes a first lens group G1 having positive refractive power, and asecond lens group G2 having negative refractive power, arranged in theorder from an object side to an image plane side. Between the secondlens group G2 and an image plane IM of an imaging element, there isprovided a filter 10. The filter 10 is omissible.

The first lens group G1 includes a first lens L1 having positiverefractive power, an aperture stop ST, a second lens L2 having positiverefractive power, and a third lens L3 having negative refractive power,arranged in the order from the object side. According to the imaginglens of the embodiment, the aperture stop ST is provided on an imageplane-side surface of the first lens L1. Here, where to provide theaperture stop ST is not limited to between the first lens L1 and thesecond lens L2 as in the imaging lens of Numerical Data Example 1.

For example, the aperture stop ST can be provided on the object side ofthe first lens L1. In case of a so-called “front aperture”-type lensconfiguration, in which the aperture stop ST is disposed on the objectside of the first lens L1, it is achievable to improve assemblingperformance of the imaging lens and reduce the manufacturing cost. Incase of the front aperture-type lens configuration, it is alsorelatively easy to shorten a total track length of the imaging lens, sothat such lens configuration is effective for mounting in a portabledevice such as cellular phones and smartphones that are popular in theseyears.

On the other hand, in case of a so-called “mid aperture”-type lensconfiguration, in which the aperture stop ST is disposed between thefirst lens L1 and the second lens L2 as in Numerical Data Example 1, aneffective diameter of the first lens L1 relative to the total tracklength of the imaging lens is large. Therefore, visual impact of theimaging lens in a camera is emphasized, so that it is possible to appealluxuriousness, high lens performance, etc. to users as a part of designof the camera.

In the first lens group G1, the first lens L1 is formed in a shape, suchthat a curvature radius r1 of an object-side surface thereof and acurvature radius r2 of an image plane-side surface thereof are bothpositive, so as to have a shape of a meniscus lens directing a convexsurface thereof to the object side near the optical axis X. The shape ofthe first lens L1 is not limited to the one in Numerical Data Example 1.The first lens L1 can be formed in any shape as long as the curvatureradius r1 of the object-side surface is positive. More specifically, thefirst lens L1 can be also formed in a shape such that the curvatureradius r2 is negative, so as to have a shape of a biconvex lens near anoptical axis X. Here, in order to more effectively attain downsizing ofthe imaging lens, the first lens L1 is preferably formed so as to have ashape of a meniscus lens directing a convex surface thereof to theobject side near the optical axis X.

The second lens L2 is formed in a shape such that a curvature radius r3of an object-side surface thereof is positive and a curvature radius r4of an image plane-side surface thereof is negative, so as to have ashape of a biconvex lens near the optical axis X. The shape of thesecond lens L2 is not limited to the one in Numerical Data Example 1.The second lens L2 can be formed in any shape as long as the curvatureradius r4 of the image plane-side surface is negative. Morespecifically, the second lens L2 can be also formed in a shape such thatthe curvature radius r3 of the object-side surface thereof is negative,so as to have a shape of a meniscus lens directing a concave surfacethereof to the object side near the optical axis X. Here, generallyspeaking, when the first lens L1 is formed so as to have a shape of abiconvex lens near the optical axis X, the second lens L2 is preferablyformed so as to have a shape of a meniscus lens directing a concavesurface thereof to the object side near the optical axis X.

The third lens L3 is formed in a shape such that a curvature radius r5of an object-side surface thereof and a curvature radius r6 of an imageplane-side surface thereof are both positive, so as to have a shape of ameniscus lens directing a convex surface thereof to the object side nearthe optical axis X. The shape of the third lens L3 is not limited to theone in Numerical Data Example 1 and can be any as long as the curvatureradius r6 of the image plane-side surface thereof is positive. Morespecifically, the third lens L3 can be also formed in a shape, such thatthe curvature radius r5 of the object-side surface thereof is negative,i.e., a shape of a biconcave lens near the optical axis X.

The second lens group G2 includes a fourth lens L4 having positiverefractive power, a fifth lens L5 having negative or positive refractivepower, and a sixth lens L6 having negative refractive power, arranged inthe order from the object side.

In the second lens group G2, the fourth lens L4 is formed in a shapesuch that a curvature radius r7 of an object-side surface thereof and acurvature radius r8 of an image plane-side surface thereof are bothnegative, so as to have a meniscus lens directing a concave surfacethereof to the object side near the optical axis X. The fifth lens L5 isformed in a shape such that a curvature radius r9 of an object-sidesurface thereof and a curvature radius r10 of an image plane-sidesurface thereof are both positive, so as to have a shape of a meniscuslens directing a convex surface thereof to the object side near theoptical axis X. The fifth lens L5 has the weakest refractive power inthe second lens group G2. The imaging lenses in Numerical Data Examples1 to 6 are examples of a lens configuration, in which the fifth lens L5has negative refractive power. The imaging lens in Numerical DataExample 7 is an example, in which the fifth lens L5 has positiverefractive power.

The sixth lens L6 is formed in a shape, such that a curvature radius r11of an object-side surface thereof and a curvature radius r12 of an imageplane-side surface thereof are both negative, so as to have a shape of ameniscus lens directing a concave surface thereof to the object sidenear the optical axis X. The shape of the sixth lens L6 is not limitedto the one in Numerical Data Example 1, and can be any as long as thecurvature radius r11 of the object-side surface thereof is negative.More specifically, the sixth lens L6 can be also formed in a shape suchthat the curvature radius r12 is positive, so as to have a shape of abiconcave lens near the optical axis X.

The fifth lens L5 and the sixth lens L6 are both formed such that theobject-side surfaces thereof and the image plane-side surfaces thereofare formed as both aspheric surfaces, and the positive refractive powersthereof become stronger towards the lens peripheries. Due to the shapesof the fifth lens L5 and the sixth lens L6, it is achievable tosatisfactorily correct off-axis chromatic aberrations of magnificationas well as axial chromatic aberrations, and also achievable to suitablyrestrain the incident angles of light beams emitted from the imaginglens to the image plane IM within the range of chief ray angle (CRA).

According to the embodiment, the imaging lens satisfies the followingconditional expressions (1) to (16):10<f1/f<40  (1)40<νd1<75  (2)40<νd2<75  (3)15<νd3<35  (4)0.7<F1/f<1.2  (5)0.3<f2/f<0.9  (6)−5<f3/f2<−1  (7)−12<F2/F1<−1.5  (8)0.1<D34/f<0.4  (9)40<νd4<75  (10)15<νd5<35  (11)40<νd6<75  (12)−15<f5/f4<−5  (13)−3<f56/f<−0.8  (14)0.7<f6/f56<1.2  (15)−3.5<f6/f<−0.5  (16)

In the above conditional expressions:

f: Focal length of the whole lens system

F1: Focal length of the first lens group G1

F2: Focal length of the second lens group G2

f1: Focal length of the first lens L1

f2: Focal length of the second lens L2

f3: Focal length of the third lens L3

f4: Focal length of the fourth lens L4

f5: Focal length of the fifth lens L5

f6: Focal length of the sixth lens L6

f56: Composite focal length of the fifth lens L5 and the sixth lens L6

D34: Distance on the optical axis X between the third lens L3 and thefourth lens L4

νd1: Abbe's number of the first lens L1

νd2: Abbe's number of the second lens L2

νd3: Abbe's number of the third lens L3

νd4: Abbe's number of the fourth lens L4

νd5: Abbe's number of the fifth lens L5

νd6: Abbe's number of the sixth lens L6

Here, it is not necessary to satisfy all of the conditional expressions,and it is achievable to obtain an effect corresponding to the respectiveconditional expression when any single one of the conditionalexpressions is individually satisfied.

In the embodiment, all lens surfaces are formed as an aspheric surface.When the aspheric shapes applied to the lens surfaces have an axis Z ina direction of the optical axis X, a height H in a directionperpendicular to the optical axis X, a conic constant k, and asphericcoefficients A₄, A₆, A₈, A₁₀, A₁₂, A₁₄, and A₁₆, the aspheric shapes ofthe lens surfaces are expressed as follows:

$\begin{matrix}{Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F-number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, r represents a curvatureradius, d represents a distance on the optical axis between lenssurfaces (surface spacing), nd represents a refractive index, and νdrepresents an Abbe's number, respectively. Here, aspheric surfaces areindicated with surface numbers i affixed with * (asterisk).

Numerical Data Example 1

Basic data are shown below. f = 4.72 mm, Fno = 2.3, ω = 36.6° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.540 0.3221.5346 56.1 (= νd1) 2* (Stop) 2.678 0.112 3* 2.631 0.705 1.5346 56.1 (=νd2) 4* −3.753 0.030 5* 13.124 0.250 1.6355 24.0 (= νd3) 6* 2.910 0.954(= D34) 7* −2.274 0.465 1.5346 56.1 (= νd4) 8* −1.665 0.128 9* 2.5940.600 1.6142 26.0 (= νd5) 10*  2.271 0.415 11*  −2.621 0.490 1.5346 56.1(= νd6) 12*  −43.371 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.818 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.594E−02,A₆ = −4.958E−03, A₈ = −6.808E−02, A₁₀ = 4.724E−02, A₁₂ = 7.098E−05, A₁₄= −1.463E−02, A₁₆ = 4.207E−03 Second Surface k = 0.000, A₄ = −8.107E−02,A₆ = −9.566E−02, A₈ = −2.004E−02, A₁₀ = 2.865E−02, A₁₂ = 5.229E−02, A₁₄= −5.644E−02, A₁₆ = 1.381E−02 Third Surface k = 0.000, A₄ = 3.973E−02,A₆ = −5.155E−02, A₈ = 1.130E−03, A₁₀ = 6.108E−03, A₁₂ = 1.955E−02, A₁₄ =−1.407E−02, A₁₆ = −9.526E−04 Fourth Surface k = 0.000, A₄ = 3.136E−02,A₆ = −2.899E−03, A₈ = 2.852E−03, A₁₀ = −1.870E−02, A₁₂ = 4.596E−03, A₁₄= −7.942E−05, A₁₆ = −5.084E−04 Fifth Surface k = 0.000, A₄ = −1.519E−01,A₆ = 2.580E−02, A₈ = −4.738E−04, A₁₀ = 1.868E−03, A₁₂ = 1.413E−02, A₁₄ =−9.547E−03, A₁₆ = 2.189E−03 Sixth Surface k = 0.000, A₄ = −1.511E−01, A₆= 4.516E−02, A₈ = −1.755E−03, A₁₀ = 2.979E−04, A₁₂ = −4.743E−03, A₁₄ =7.871E−03, A₁₆ = −2.368E−03 Seventh Surface k = 0.000, A₄ = 1.374E−01,A₆ = −9.270E−02, A₈ = 7.188E−02, A₁₀ = −7.831E−02, A₁₂ = 5.426E−02, A₁₄= −2.370E−02, A₁₆ = 3.688E−03 Eighth Surface k = 0.000, A₄ = 7.577E−02,A₆ = 2.107E−02, A₈ = −4.554E−03, A₁₀ = 2.116E−03, A₁₂ = −5.293E−03, A₁₄= 1.770E−03, A₁₆ = 2.075E−05 Ninth Surface k = 0.000, A₄ = −1.573E−01,A₆ = 3.006E−02, A₈ = 2.872E−03, A₁₀ = −7.123E−03, A₁₂ = 1.359E−03, A₁₄ =2.254E−04, A₁₆ = −5.339E−05 Tenth Surface k = 0.000, A₄ = −1.770E−01, A₆= 4.465E−02, A₈ = −1.142E−02, A₁₀ = 1.116E−03, A₁₂ = 2.618E−04, A₁₄ =−6.690E−05, A₁₆ = 3.254E−06 Eleventh Surface k = 0.000, A₄ = −1.005E−02,A₆ = 1.677E−02, A₈ = −3.286E−03, A₁₀ = 5.634E−04, A₁₂ = −7.823E−05, A₁₄= 2.687E−06, A₁₆ = 3.551E−07 Twelfth Surface k = 0.000, A₄ = −3.064E−02,A₆ = 2.210E−02, A₈ = −5.737E−03, A₁₀ = 5.547E−04, A₁₂ = 1.202E−05, A₁₄ =−5.825E−06, A₁₆ = 2.882E−07 f1 = 50.84 mm f2 = 3.01 mm f3 = −5.94 mm f4= 9.18 mm f5 = −100.87 mm f6 = −5.24 mm f56 = −5.41 mm F1 = 4.90 mm F2 =−17.03 mm The values of the respective conditional expressions are asfollows: f1/f = 10.78 F1/f = 1.04 f2/f = 0.64 f3/f2 = −1.97 F2/F1 =−3.48 D34/f = 0.20 f5/f4 = −10.99 f56/f = −1.15 f6/f56 = 0.97 f6/f =−1.11

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.53 mm, and downsizing ofthe imaging lens is attained.

FIG. 2 shows a lateral aberration that corresponds to a ratio H of eachimage height to the maximum image height (hereinafter referred to as“image height ratio H”), which is divided into a tangential directionand a sagittal direction (The same is true for FIGS. 5, 8, 11, 14, 17,and 20), in the imaging lens of Numerical Data Example 1. Furthermore,FIG. 3 shows a spherical aberration (mm), astigmatism (mm), and adistortion (%), respectively. In the astigmatism diagram, an aberrationon a sagittal image surface S and an aberration on a tangential imagesurface T are respectively indicated (The same is true for FIGS. 6, 9,12, 15, 18, and 21). As shown in FIGS. 2 and 3, according to the imaginglens of Numerical Data Example 1, the aberrations are satisfactorilycorrected.

Numerical Data Example 2

Basic data are shown below. f = 4.17 mm, Fno = 2.2, ω = 40.0° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.640 0.3011.5346 56.1 (= νd1) 2* (Stop) 2.623 0.100 3* 2.548 0.731 1.5346 56.1 (=νd2) 4* −3.600 0.030 5* 12.413 0.250 1.6355 24.0 (= νd3) 6* 3.003 0.827(= D34) 7* −2.554 0.502 1.5346 56.1 (= νd4) 8* −1.644 0.008 9* 2.5820.628 1.6142 26.0 (= νd5) 10*  2.251 0.430 11*  −2.619 0.490 1.5346 56.1(= νd6) 12*  −45.373 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.695 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −9.029E−02,A₆ = −3.009E−03, A₈ = −6.611E−02, A₁₀ = 4.776E−02, A₁₂ = −2.396E−05, A₁₄= −1.472E−02, A₁₆ = 4.271E−03 Second Surface k = 0.000, A₄ = −8.079E−02,A₆ = −9.490E−02, A₈ = −1.944E−02, A₁₀ = 2.897E−02, A₁₂ = 5.234E−02, A₁₄= −5.654E−02, A₁₆ = 1.371E−02 Third Surface k = 0.000, A₄ = 3.895E−02,A₆ = −5.510E−02, A₈ = 6.493E−05, A₁₀ = 5.932E−03, A₁₂ = 1.950E−02, A₁₄ =−1.410E−02, A₁₆ = −9.727E−04 Fourth Surface k = 0.000, A₄ = 2.831E−02,A₆ = −2.611E−03, A₈ = 3.332E−03, A₁₀ = −1.855E−02, A₁₂ = 4.623E−03, A₁₄= −6.681E−05, A₁₆ = −4.902E−04 Fifth Surface k = 0.000, A₄ = −1.498E−01,A₆ = 2.642E−02, A₈ = −6.170E−04, A₁₀ = 1.760E−03, A₁₂ = 1.409E−02, A₁₄ =−9.566E−03, A₁₆ = 2.152E−03 Sixth Surface k = 0.000, A₄ = −1.477E−01, A₆= 4.201E−02, A₈ = −2.268E−03, A₁₀ = 1.714E−04, A₁₂ = −4.848E−03, A₁₄ =7.816E−03, A₁₆ = −2.383E−03 Seventh Surface k = 0.000, A₄ = 1.460E−01,A₆ = −9.229E−02, A₈ = 7.149E−02, A₁₀ = −7.807E−02, A₁₂ = 5.476E−02, A₁₄= −2.324E−02, A₁₆ = 3.971E−03 Eighth Surface k = 0.000, A₄ = 7.413E−02,A₆ = 2.130E−02, A₈ = −4.373E−03, A₁₀ = 2.217E−03, A₁₂ = −5.217E−03, A₁₄= 1.817E−03, A₁₆ = 4.705E−05 Ninth Surface k = 0.000, A₄ = −1.645E−01,A₆ = 2.898E−02, A₈ = 1.460E−03, A₁₀ = −7.751E−03, A₁₂ = 1.221E−03, A₁₄ =2.346E−04, A₁₆ = −3.068E−05 Tenth Surface k = 0.000, A₄ = −1.742E−01, A₆= 4.358E−02, A₈ = −1.132E−02, A₁₀ = 1.136E−03, A₁₂ = 2.605E−04, A₁₄ =−6.787E−05, A₁₆ = 3.043E−06 Eleventh Surface k = 0.000, A₄ = −1.212E−02,A₆ = 1.702E−02, A₈ = −3.251E−03, A₁₀ = 5.657E−04, A₁₂ = −7.834E−05, A₁₄= 2.636E−06, A₁₆ = 3.396E−07 Twelfth Surface k = 0.000, A₄ = −3.082E−02,A₆ = 2.153E−02, A₈ = −5.713E−03, A₁₀ = 5.587E−04, A₁₂ = 1.227E−05, A₁₄ =−5.832E−06, A₁₆ = 2.817E−07 f1 = 145.98 mm f2 = 2.91 mm f3 = −6.30 mm f4= 7.23 mm f5 = −102.11 mm f6 = −5.22 mm f56 = −5.42 mm F1 = 4.72 mm F2 =−46.16 mm The values of the respective conditional expressions are asfollows: f1/f = 34.98 F1/f = 1.13 f2/f = 0.70 f3/f2 = −2.16 F2/F1 =−9.78 D34/f = 0.20 f5/f4 = −14.11 f56/f = −1.30 f6/f56 = 0.96 f6/f =−1.25

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.23 mm, and downsizing ofthe imaging lens is attained.

FIG. 5 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 6 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 2. As shown in FIGS. 5 and 6,according to the imaging lens of Numerical Data Example 2, theaberrations are also satisfactorily corrected.

Numerical Data Example 3

Basic data are shown below. f = 5.57 mm, Fno = 3.0, ω = 32.1° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.411 0.3101.5346 56.1 (= νd1) 2* (Stop) 2.411 0.120 3* 2.584 0.786 1.5346 56.1 (=νd2) 4* −3.786 0.030 5* 13.454 0.250 1.6355 24.0 (= νd3) 6* 2.712 0.651(= D34) 7* −2.286 0.619 1.5346 56.1 (= νd4) 8* −1.734 0.066 9* 2.7360.534 1.6142 26.0 (= νd5) 10*  2.425 0.377 11*  −2.777 0.490 1.5346 56.1(= νd6) 12*  −10.661 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 1.829 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.574E−02,A₆ = −2.408E−03, A₈ = −6.584E−02, A₁₀ = 4.778E−02, A₁₂ = −1.745E−04, A₁₄= −1.483E−02, A₁₆ = 4.339E−03 Second Surface k = 0.000, A₄ = −8.011E−02,A₆ = −9.318E−02, A₈ = −1.939E−02, A₁₀ = 2.819E−02, A₁₂ = 5.150E−02, A₁₄= −5.701E−02, A₁₆ = 1.391E−02 Third Surface k = 0.000, A₄ = 3.737E−02,A₆ = −5.662E−02, A₈ = 9.497E−04, A₁₀ = 6.629E−03, A₁₂ = 1.907E−02, A₁₄ =−1.547E−02, A₁₆ = −2.843E−03 Fourth Surface k = 0.000, A₄ = 3.028E−02,A₆ = −4.810E−03, A₈ = 1.480E−03, A₁₀ = −1.931E−02, A₁₂ = 4.487E−03, A₁₄= −1.826E−05, A₁₆ = −4.979E−04 Fifth Surface k = 0.000, A₄ = −1.489E−01,A₆ = 2.903E−02, A₈ = 8.808E−04, A₁₀ = 2.428E−03, A₁₂ = 1.435E−02, A₁₄ =−9.417E−03, A₁₆ = 2.330E−03 Sixth Surface k = 0.000, A₄ = −1.486E−01, A₆= 3.921E−02, A₈ = −3.126E−03, A₁₀ = 3.864E−05, A₁₂ = −4.673E−03, A₁₄ =8.094E−03, A₁₆ = −2.132E−03 Seventh Surface k = 0.000, A₄ = 1.493E−01,A₆ = −9.462E−02, A₈ = 6.796E−02, A₁₀ = −7.982E−02, A₁₂ = 5.416E−02, A₁₄= −2.336E−02, A₁₆ = 3.969E−03 Eighth Surface k = 0.000, A₄ = 7.146E−02,A₆ = 2.321E−02, A₈ = −3.123E−03, A₁₀ = 2.538E−03, A₁₂ = −5.186E−03, A₁₄= 1.796E−03, A₁₆ = 2.765E−05 Ninth Surface k = 0.000, A₄ = −1.612E−01,A₆ = 2.896E−02, A₈ = 2.664E−03, A₁₀ = −7.024E−03, A₁₂ = 1.339E−03, A₁₄ =1.967E−04, A₁₆ = −6.638E−05 Tenth Surface k = 0.000, A₄ = −1.729E−01, A₆= 4.397E−02, A₈ = −1.147E−02, A₁₀ = 1.109E−03, A₁₂ = 2.599E−04, A₁₄ =−6.764E−05, A₁₆ = 2.959E−06 Eleventh Surface k = 0.000, A₄ = −1.458E−02,A₆ = 1.675E−02, A₈ = −3.260E−03, A₁₀ = 5.684E−04, A₁₂ = −7.722E−05, A₁₄= 2.945E−06, A₁₆ = 4.082E−07 Twelfth Surface k = 0.000, A₄ = −2.949E−02,A₆ = 2.146E−02, A₈ = −5.720E−03, A₁₀ = 5.604E−04, A₁₂ = 1.266E−05, A₁₄ =−5.799E−06, A₁₆ = 2.795E−07 f1 = 100.56 mm f2 = 3.00 mm f3 = −5.39 mm f4= 9. 66 mm f5 = −100.49 mm f6 = −7.18 mm f56 = −7.18 mm F1 = 5.45 mm F2= −32.86 mm The values of the respective conditional expressions are asfollows: f1/f = 18.04 F1/f = 0.98 f2/f = 0.54 f3/f2 = −1.80 F2/F1 =−6.03 D34/f = 0.18 f5/f4 = −10.40 f56/f = −1.29 f6/f56 = 1.00 f6/f =−1.29

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 6.30 mm, and downsizing ofthe imaging lens is attained.

FIG. 8 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 9 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 3. As shown in FIGS. 8 and 9,according to the imaging lens of Numerical Data Example 3, theaberrations are also satisfactorily corrected.

Numerical Data Example 4

Basic data are shown below. f = 5.49 mm, Fno = 2.7, ω = 32.5° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.919 0.3111.5346 56.1 (= νd1) 2* (Stop) 2.972 0.059 3* 3.227 0.597 1.5346 56.1 (=νd2) 4* −3.911 0.030 5* 11.449 0.250 1.6355 24.0 (= νd3) 6* 3.477 1.824(= D34) 7* −2.256 0.328 1.5346 56.1 (= νd4) 8* −1.706 0.010 9* 2.5680.520 1.6142 26.0 (= νd5) 10*  2.272 0.420 11*  −2.623 0.490 1.5346 56.1(= νd6) 12*  −108.154 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.915 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.446E−02,A₆ = 2.265E−03, A₈ = −6.239E−02, A₁₀ = 4.824E−02, A₁₂ = −3.062E−04, A₁₄= −1.524E−02, A₁₆ = 4.143E−03 Second Surface k = 0.000, A₄ = −7.225E−02,A₆ = −8.484E−02, A₈ = −1.710E−02, A₁₀ = 2.743E−02, A₁₂ = 5.023E−02, A₁₄= −5.745E−02, A₁₆ = 1.454E−02 Third Surface k = 0.000, A₄ = 4.777E−02,A₆ = −5.361E−02, A₈ = 1.061E−03, A₁₀ = 6.257E−03, A₁₂ = 1.947E−02, A₁₄ =−1.491E−02, A₁₆ = −2.018E−03 Fourth Surface k = 0.000, A₄ = 2.522E−02,A₆ = −1.962E−03, A₈ = 5.473E−03, A₁₀ = −1.738E−02, A₁₂ = 4.681E−03, A₁₄= −7.421E−04, A₁₆ = −1.111E−03 Fifth Surface k = 0.000, A₄ = −1.372E−01,A₆ = 2.810E−02, A₈ = −1.443E−03, A₁₀ = 9.473E−04, A₁₂ = 1.368E−02, A₁₄ =−9.705E−03, A₁₆ = 1.879E−03 Sixth Surface k = 0.000, A₄ = −1.405E−01, A₆= 5.035E−02, A₈ = −5.014E−04, A₁₀ = 1.135E−04, A₁₂ = −5.309E−03, A₁₄ =7.228E−03, A₁₆ = −2.740E−03 Seventh Surface k = 0.000, A₄ = 1.023E−01,A₆ = −8.409E−02, A₈ = 7.439E−02, A₁₀ = −7.942E−02, A₁₂ = 5.363E−02, A₁₄= −2.355E−02, A₁₆ = 4.053E−03 Eighth Surface k = 0.000, A₄ = 7.354E−02,A₆ = 1.479E−02, A₈ = −6.064E−03, A₁₀ = 1.766E−03, A₁₂ = −5.416E−03, A₁₄= 1.719E−03, A₁₆ = 3.610E−06 Ninth Surface k = 0.000, A₄ = −1.471E−01,A₆ = 3.398E−02, A₈ = 2.887E−03, A₁₀ = −7.167E−03, A₁₂ = 1.329E−03, A₁₄ =2.112E−04, A₁₆ = −5.826E−05 Tenth Surface k = 0.000, A₄ = −1.794E−01, A₆= 4.560E−02, A₈ = −1.140E−02, A₁₀ = 1.081E−03, A₁₂ = 2.544E−04, A₁₄ =−6.744E−05, A₁₆ = 3.371E−06 Eleventh Surface k = 0.000, A₄ = −1.180E−02,A₆ = 1.648E−02, A₈ = −3.280E−03, A₁₀ = 5.656E−04, A₁₂ = −7.795E−05, A₁₄= 2.670E−06, A₁₆ = 3.494E−07 Twelfth Surface k = 0.000, A₄ = −2.650E−02,A₆ = 2.137E−02, A₈ = −5.791E−03, A₁₀ = 5.614E−04, A₁₂ = 1.252E−05, A₁₄ =−5.831E−06, A₁₆ = 2.817E−07 f1 = 100.47 mm f2 = 3.41 mm f3 = −7.95 mm f4= 10.84 mm f5 = −96.67 mm f6 = −5.04 mm f56 = −5.14 mm F1 = 5.31 mm F2 =−11.14 mm The values of the respective conditional expressions are asfollows: f1/f = 18.30 F1/f =0.97 f2/f = 0.62 f3/f2 = −2.33 F2/F1 = −2.10D34/f = 0.33 f5/f4 = −8.91 f56/f = −0.94 f6/f56 = 0.98 f6/f = −0.92

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.99 mm, and downsizing ofthe imaging lens is attained.

FIG. 11 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 12 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 4. As shown in FIGS. 11 and 12,according to the imaging lens of Numerical Data Example 4, theaberrations are also satisfactorily corrected.

Numerical Data Example 5

Basic data are shown below. f = 4.78 mm, Fno = 2.3, ω = 36.2° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.820 0.3911.5346 56.1 (= νd1) 2* (Stop) 2.832 0.101 3* 3.266 0.626 1.5346 56.1 (=νd2) 4* −4.128 0.030 5* 8.443 0.250 1.6355 24.0 (= νd3) 6* 4.195 1.266(= D34) 7* −2.233 0.373 1.5346 56.1 (= νd4) 8* −1.728 0.010 9* 2.5980.518 1.6142 26.0 (= νd5) 10*  2.263 0.425 11*  −2.612 0.490 1.5346 56.1(= νd6) 12*  −325.335 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.803 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.756E−02,A₆ = 6.966E−03, A₈ = −6.001E−02, A₁₀ = 4.784E−02, A₁₂ = −9.244E−04, A₁₄= −1.503E−02, A₁₆ = 5.154E−03 Second Surface k = 0.000, A₄ = −7.256E−02,A₆ = −8.029E−02, A₈ = −1.447E−02, A₁₀ = 2.822E−02, A₁₂ = 5.017E−02, A₁₄= −5.749E−02, A₁₆ = 1.506E−02 Third Surface k = 0.000, A₄ = 5.347E−02,A₆ = −6.030E−02, A₈ = −1.115E−04, A₁₀ = 7.713E−03, A₁₂ = 2.072E−02, A₁₄= −1.472E−02, A₁₆ = −2.467E−03 Fourth Surface k = 0.000, A₄ = 1.710E−02,A₆ = −1.536E−03, A₈ = 5.693E−03, A₁₀ = −1.730E−02, A₁₂ = 4.608E−03, A₁₄= −8.975E−04, A₁₆ = −1.305E−03 Fifth Surface k = 0.000, A₄ = −1.388E−01,A₆ = 2.945E−02, A₈ = −1.784E−03, A₁₀ = 5.741E−04, A₁₂ = 1.364E−02, A₁₄ =−9.545E−03, A₁₆ = 2.093E−03 Sixth Surface k = 0.000, A₄ = −1.348E−01, A₆= 4.218E−02, A₈ = −7.409E−04, A₁₀ = 1.474E−03, A₁₂ = −4.458E−03, A₁₄ =7.595E−03, A₁₆ = −2.580E−03 Seventh Surface k = 0.000, A₄ = 1.215E−01,A₆ = −9.862E−02, A₈ = 7.588E−02, A₁₀ = −7.883E−02, A₁₂ = 5.363E−02, A₁₄= −2.323E−02, A₁₆ = 4.465E−03 Eighth Surface k = 0.000, A₄ = 6.557E−02,A₆ = 1.334E−02, A₈ = −6.731E−03, A₁₀ = 2.039E−03, A₁₂ = −5.180E−03, A₁₄= 1.818E−03, A₁₆ = 3.540E−05 Ninth Surface k = 0.000, A₄ = −1.767E−01,A₆ = 3.036E−02, A₈ = 3.829E−03, A₁₀ = −7.507E−03, A₁₂ = 1.041E−03, A₁₄ =1.348E−04, A₁₆ = −4.949E−05 Tenth Surface k = 0.000, A₄ = −1.823E−01, A₆= 4.478E−02, A₈ = −1.096E−02, A₁₀ = 1.154E−03, A₁₂ = 2.537E−04, A₁₄ =−6.993E−05, A₁₆ = 2.705E−06 Eleventh Surface k = 0.000, A₄ = −9.793E−03,A₆ = 1.721E−02, A₈ = −3.290E−03, A₁₀ = 5.562E−04, A₁₂ = −7.907E−05, A₁₄= 2.557E−06, A₁₆ = 3.473E−07 Twelfth Surface k = 0.000, A₄ = −3.084E−02,A₆ = 2.134E−02, A₈ = −5.751E−03, A₁₀ = 5.647E−04, A₁₂ = 1.259E−05, A₁₄ =−5.851E−06, A₁₆ = 2.761E−07 f1 = 100.58 mm f2 = 3.51 mm f3 = −13.43 mmf4 = 11.38 mm f5 = −69.04 mm f6 = −4.93 mm f56 = −4.92 mm F1 = 4.52 mmF2 = −9.48 mm The values of the respective conditional expressions areas follows: f1/f = 21.02 F1/f =0.94 f2/f = 0.73 f3/f2 = −3.82 F2/F1 =−2.10 D34/f = 0.26 f5/f4 = −6.07 f56/f = −1.03 f6/f56 = 1.00 f6/f =−1.03

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.52 mm, and downsizing ofthe imaging lens is attained.

FIG. 14 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 15 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 5. As shown in FIGS. 14 and 15,according to the imaging lens of Numerical Data Example 5, theaberrations are also satisfactorily corrected.

Numerical Data Example 6

Basic data are shown below. f = 4.59 mm, Fno = 2.3, ω = 37.3° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.394 0.3241.5346 56.1 (= νd1) 2* (Stop) 2.506 0.078 3* 2.498 0.778 1.5346 56.1 (=νd2) 4* −4.033 0.030 5* 9.929 0.250 1.6355 24.0 (= νd3) 6* 2.749 0.751(= D34) 7* −1.844 0.401 1.5346 56.1 (= νd4) 8* −1.669 0.232 9* 2.5800.688 1.6142 26.0 (= νd5) 10*  2.218 0.581 11*  −2.659 0.490 1.5346 56.1(= νd6) 12*  −4.576 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 0.646 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −7.447E−02,A₆ = 2.499E−03, A₈ = −7.205E−02, A₁₀ = 4.892E−02, A₁₂ = −5.711E−04, A₁₄= −1.577E−02, A₁₆ = 5.190E−03 Second Surface k = 0.000, A₄ = −6.909E−02,A₆ = −9.439E−02, A₈ = −1.939E−02, A₁₀ = 2.926E−02, A₁₂ = 5.104E−02, A₁₄= −5.806E−02, A₁₆ = 1.510E−02 Third Surface k = 0.000, A₄ = 2.859E−02,A₆ = −5.882E−02, A₈ = 3.748E−03, A₁₀ = 1.056E−02, A₁₂ = 1.872E−02, A₁₄ =−1.482E−02, A₁₆ = −1.925E−03 Fourth Surface k = 0.000, A₄ = 8.676E−03,A₆ = −2.236E−03, A₈ = 5.505E−03, A₁₀ = −1.880E−02, A₁₂ = 3.832E−03, A₁₄= −7.443E−04, A₁₆ = −4.898E−04 Fifth Surface k = 0.000, A₄ = −1.455E−01,A₆ = 2.401E−02, A₈ = −6.875E−03, A₁₀ = 5.477E−04, A₁₂ = 1.407E−02, A₁₄ =−8.432E−03, A₁₆ = 1.879E−03 Sixth Surface k = 0.000, A₄ = −1.253E−01, A₆= 2.536E−02, A₈ = 1.986E−03, A₁₀ = −3.142E−04, A₁₂ = −7.558E−03, A₁₄ =7.643E−03, A₁₆ = −1.722E−03 Seventh Surface k = 0.000, A₄ = 1.443E−01,A₆ = −1.142E−01, A₈ = 8.684E−02, A₁₀ = −7.662E−02, A₁₂ = 4.838E−02, A₁₄= −2.309E−02, A₁₆ = 4.006E−03 Eighth Surface k = 0.000, A₄ = 3.238E−02,A₆ = 2.005E−02, A₈ = −4.774E−03, A₁₀ = 3.102E−03, A₁₂ = −5.191E−03, A₁₄= 2.182E−03, A₁₆ = 1.113E−04 Ninth Surface k = 0.000, A₄ = −1.806E−01,A₆ = 3.290E−02, A₈ = 2.012E−03, A₁₀ = −8.917E−03, A₁₂ = 1.206E−03, A₁₄ =2.038E−04, A₁₆ = −2.055E−05 Tenth Surface k = 0.000, A₄ = −1.786E−01, A₆= 4.410E−02, A₈ = −1.250E−02, A₁₀ = 1.273E−03, A₁₂ = 2.399E−04, A₁₄ =−7.472E−05, A₁₆ = 5.332E−06 Eleventh Surface k = 0.000, A₄ = −1.867E−02,A₆ = 1.627E−02, A₈ = −2.858E−03, A₁₀ = 4.966E−04, A₁₂ = −7.390E−05, A₁₄= 2.900E−06, A₁₆ = 2.802E−07 Twelfth Surface k = 0.000, A₄ = −1.489E−02,A₆ = 2.173E−02, A₈ = −5.825E−03, A₁₀ = 5.278E−04, A₁₂ = 2.004E−05, A₁₄ =−6.555E−06, A₁₆ = 3.081E−07 f1 = 49.97 mm f2 = 3.01 mm f3 = −6.06 mm f4= 18.28 mm f5 = −93.19 mm f6 = −13.03 mm f56 = −12.54 mm F1 = 4.76 mm F2= −46.78 mm The values of the respective conditional expressions are asfollows: f1/f = 10.88 F1/f = 1.04 f2/f =0.66 f3/f2 = −2.01 F2/F1 = −9.82D34/f = 0.16 f5/f4 = −5.10 f56/f = −2.73 f6/f56 = 1.04 f6/f = −2.84

Accordingly, the imaging lens of Numerical Data Example 6 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 5.49 mm, and downsizing ofthe imaging lens is attained.

FIG. 17 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 18 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 6. As shown in FIGS. 17 and 18,according to the imaging lens of Numerical Data Example 6, theaberrations are also satisfactorily corrected.

Numerical Data Example 7

Basic data are shown below. f = 5.59 mm, Fno = 3.0, ω = 32.1° Unit: mmSurface Data Surface Number i r d nd νd (Object) ∞ ∞ 1* 2.543 0.3431.5346 56.1 (= νd1) 2* (Stop) 2.557 0.108 3* 2.573 0.693 1.5346 56.1 (=νd2) 4* −3.368 0.046 5* 24.576 0.250 1.6355 24.0 (= νd3) 6* 3.170 0.674(= D34) 7* −1.708 0.423 1.5346 56.1 (= νd4) 8* −1.798 0.348 9* 2.5350.509 1.6142 26.0 (= νd5) 10*  2.440 0.544 11*  −2.749 0.405 1.5346 56.1(= νd6) 12*  −6.157 0.040 13  ∞ 0.300 1.5168 64.2 14  ∞ 1.484 (Image ∞plane) Aspheric Surface Data First Surface k = 0.000, A₄ = −8.425E−02,A₆ = −5.731E−03, A₈ = −6.587E−02, A₁₀ = 5.014E−02, A₁₂ = −1.499E−03, A₁₄= −1.475E−02, A₁₆ = 4.102E−03 Second Surface k = 0.000, A₄ = −8.546E−02,A₆ = −9.353E−02, A₈ = −1.739E−02, A₁₀ = 2.901E−02, A₁₂ = 5.224E−02, A₁₄= −5.801E−02, A₁₆ = 1.399E−02 Third Surface k = 0.000, A₄ = 3.398E−02,A₆ = −5.949E−02, A₈ = 3.317E−03, A₁₀ = 6.791E−03, A₁₂ = 1.762E−02, A₁₄ =−1.621E−02, A₁₆ = −2.591E−03 Fourth Surface k = 0.000, A₄ = 2.275E−02,A₆ = −4.423E−03, A₈ = 1.566E−03, A₁₀ = −2.000E−02, A₁₂ = 5.082E−03, A₁₄= 5.211E−04, A₁₆ = −1.366E−04 Fifth Surface k = 0.000, A₄ = −1.541E−01,A₆ = 3.068E−02, A₈ = 2.597E−03, A₁₀ = 3.400E−03, A₁₂ = 1.510E−02, A₁₄ =−8.697E−03, A₁₆ = 3.304E−03 Sixth Surface k = 0.000, A₄ = −1.499E−01, A₆= 3.668E−02, A₈ = −2.460E−03, A₁₀ = 3.961E−04, A₁₂ = −4.229E−03, A₁₄ =7.933E−03, A₁₆ = −1.968E−03 Seventh Surface k = 0.000, A₄ = 1.518E−01,A₆ = −9.269E−02, A₈ = 6.624E−02, A₁₀ = −7.946E−02, A₁₂ = 5.477E−02, A₁₄= −2.393E−02, A₁₆ = 3.482E−03 Eighth Surface k = 0.000, A₄ = 6.958E−02,A₆ = 2.231E−02, A₈ = −2.553E−03, A₁₀ = 3.319E−03, A₁₂ = −5.110E−03, A₁₄= 1.803E−03, A₁₆ = 2.276E−05 Ninth Surface k = 0.000, A₄ = −1.701E−01,A₆ = 3.174E−02, A₈ = 2.149E−03, A₁₀ = −7.002E−03, A₁₂ = 1.417E−03, A₁₄ =2.060E−04, A₁₆ = −6.997E−05 Tenth Surface k = 0.000, A₄ = −1.692E−01, A₆= 4.425E−02, A₈ = −1.136E−02, A₁₀ = 1.114E−03, A₁₂ = 2.619E−04, A₁₄ =−6.733E−05, A₁₆ = 3.353E−06 Eleventh Surface k = 0.000, A₄ = −1.720E−02,A₆ = 1.657E−02, A₈ = −3.265E−03, A₁₀ = 5.669E−04, A₁₂ = −7.870E−05, A₁₄= 2.750E−06, A₁₆ = 3.348E−07 Twelfth Surface k = 0.000, A₄ = −2.717E−02,A₆ = 2.082E−02, A₈ = −5.708E−03, A₁₀ = 5.634E−04, A₁₂ = 1.272E−05, A₁₄ =−5.969E−06, A₁₆ = 2.546E−07 f1 = 90.92 mm f2 = 2.84 mm f3 = −5.75 mm f4= 100.94 mm f5 = 101.70 mm f6 = −9.69 mm f56 = −11.75 mm F1 = 4.78 mm F2= −12.37 mm The values of the respective conditional expressions are asfollows: f1/f = 16.27 F1/f = 0.86 f2/f = 0.51 f3/f2 = −2.02 F2/F1 =−2.59 D34/f = 0.12 f5/f4 = 1.01 f56/f = −2.10 f6/f56 = 0.83 f6/f = −1.73

Accordingly, the imaging lens of Numerical Data Example 7 satisfies theabove-described conditional expressions. The distance on the opticalaxis X from the object-side surface of the first lens L1 to the imageplane IM (length without the filter 10) is 6.06 mm, and downsizing ofthe imaging lens is attained.

FIG. 20 shows a lateral aberration that corresponds to the image heightratio H of the imaging lens, and FIG. 21 shows a spherical aberration(mm), astigmatism (mm), and a distortion (%), respectively, of theimaging lens of Numerical Data Example 7. As shown in FIGS. 20 and 21,according to the imaging lens of Numerical Data Example 7, theaberrations are also satisfactorily corrected.

According to the imaging lens of the embodiment described above, it isachievable to have a wide angle of view (2ω) of 80° or greater.According to Numerical Data Examples 1 to 7, the imaging lenses havewide angles of view of 64.2° to 80.0°. According to the imaging lens ofthe embodiment, it is possible to take an image over a wider range thanthat can be taken by a conventional imaging lens.

Moreover, in these years, with advancement in digital zoom technology,which enables to enlarge any area of an image obtained through animaging lens by image processing, an imaging element having a high pixelcount is often used in combination with a high resolution imaging lens.In case of such an imaging element with a high pixel count, alight-receiving area of each pixel decreases, so that an image takentends to be dark. As a method for correcting this problem, there is amethod of enhancing light-receiving sensitivity of the imaging elementusing an electrical circuit. However, when the light-receivingsensitivity increases, a noise component that does not directlycontribute to image formation is also amplified, so that it is necessaryto use another circuit for reducing the noise. According to the imaginglenses of Numerical Data Examples 1 to 7, the Fnos are as small as 2.2to 3.0. According to the imaging lens of the embodiment, it is possibleto obtain a sufficiently bright image without the above-describedelectrical circuit.

Accordingly, when the imaging lens of the embodiment is mounted in animaging optical system such as cameras to be built in portable devicesincluding cellular phones, portable information terminals, andsmartphones, digital still cameras, security cameras, onboard cameras,and network cameras, it is possible to attain both high performance anddownsizing of the cameras.

The present invention is applicable in an imaging lens to be mounted ina relatively small camera for portable devices including cellularphones, smartphones, and portable information terminals, digital stillcameras, security cameras, onboard cameras, and network cameras.

The disclosure of Japanese Patent Application No. 2014-136541, filed onJul. 2, 2014, is incorporated in the application by reference.

While the present invention has been explained with reference to thespecific embodiment of the present invention, the explanation isillustrative and the present invention is limited only by the appendedclaims.

What is claimed is:
 1. An imaging lens comprising: a first lens group;and a second lens group, arranged in this order from an object side toan image plane side, wherein said first lens group includes a first lenshaving positive refractive power, a second lens having positiverefractive power, and a third lens, arranged with a space in between,said second lens group includes a fourth lens having positive refractivepower, a fifth lens, and a sixth lens having negative refractive power,arranged with a space in between, said second lens is formed in a shapeso that a curvature radius of a surface thereof on the image plane sideis negative near an optical axis thereof, said third lens is formed in ashape so that a curvature radius of a surface thereof on the image planeside is positive near an optical axis thereof, said third lens is formedin the shape so that a surface thereof on the object side and thesurface thereof on the image plane side are aspheric, said fourth lensis formed in a shape so that a curvature radius of a surface thereof onthe object side is negative near an optical axis thereof, said fifthlens is formed in a shape so that a surface thereof on the object sideand a surface thereof on the image plane side are aspheric, said sixthlens is formed in a shape so that a surface thereof on the object sideand a surface thereof on the image plane side are aspheric, and saidfifth lens has an Abbe's number νd5 so that the following conditionalexpression is satisfied:15<νd5<35.
 2. The imaging lens according to claim 1, wherein said firstlens group has positive refractive power, and said second lens group hasnegative refractive power.
 3. The imaging lens according to claim 1,wherein said third lens has negative refractive power.
 4. The imaginglens according to claim 1, wherein said first lens is formed in a shapeso that a curvature radius of a surface thereof on the object side and acurvature radius of a surface thereof on the image plane side arepositive near an optical axis thereof.
 5. The imaging lens according toclaim 1, wherein said second lens is formed in the shape so that acurvature radius of a surface thereof on the object side is positivenear the optical axis thereof.
 6. The imaging lens according to claim 1,wherein said third lens is formed in the shape so that a curvatureradius of the surface thereof on the object side is positive near theoptical axis thereof.
 7. The imaging lens according to claim 1, whereinsaid fourth lens is formed in the shape so that a curvature radius of asurface thereof on the image plane side is negative near the opticalaxis thereof.
 8. The imaging lens according to claim 1, wherein saidfifth lens is formed in the shape so that a curvature radius of thesurface thereof on the object side and a curvature radius of the surfacethereof on the image plane side are positive near an optical axisthereof.
 9. The imaging lens according to claim 1, wherein said sixthlens is formed in the shape so that a curvature radius of the surfacethereof on the object side and a curvature radius of the surface thereofon the image plane side are negative near an optical axis thereof. 10.The imaging lens according to claim 1, wherein said first lens has afocal length f1, said first lens has an Abbe's number νd1, said secondlens has an Abbe's number νd2, and said third lens has an Abbe's numberνd3 so that the following conditional expressions are satisfied:10<f1/f<4040<νd1<7540<νd2<7515<νd3<35 where f is a focal length of a whole lens system.
 11. Theimaging lens according to claim 1, wherein said second lens has a focallength f2, and said third lens has a focal length f3 so that thefollowing conditional expression is satisfied:−5<f3/f2<−1.
 12. The imaging lens according to claim 1, wherein saidfirst lens group has a focal length F1 and said second lens group has afocal length F2 so that the following conditional expression issatisfied:−12<F2/F1<−1.5.
 13. The imaging lens according to claim 1, wherein saidthird lens is arranged away from the fourth lens by a distance D34 onthe optical axis so that the following conditional expression issatisfied:0.1<D34/f<0.4 where f is a focal length of a whole lens system.
 14. Theimaging lens according to claim 1, wherein said fifth lens has negativerefractive power, and said fourth lens has a focal length f4 and saidfifth lens has a focal length f5 so that the following conditionalexpression is satisfied:−15<f5/f4<−5.
 15. The imaging lens according to claim 1, wherein saidfifth lens and said sixth lens have a composite focal length f56 so thatthe following conditional expression is satisfied:−3<f56/f<−0.8 where f is a focal length of a whole lens system.